We are studying murine intracisternal A-particles as a model for the evolution and expression of integrated reiterated viral genes. Multiple individual A-particle genes have been isolated from mouse DNA recombinant DNA techniques. These genes, which number 1000-2000 copies per cell, make up a family of homologous but non-identical variants which are highly conserved in various strains and tissues of Mus musculus. The region of sequence homology between A-particles and the infectious extracellular M432 retrovirus of Mus cervicolor has been defined using cloned examples of the two types of genes.